Olefin polymerization processes may be conducted under slurry conditions. Such polymerization processes may be carried out in a loop reactor in which monomers are polymerized to form a slurry of solid polymer particles in a liquid medium. Portions of the slurry are withdrawn from the loop reactor through take-offs devices such as settling legs and continuous take-offs.
The slurry is then processed to separate the liquid medium (for example, a liquid diluent) from the solid polymer particles. Flashline heaters and flash chambers have been used to vaporize the liquid diluent through a combination of temperature and pressure adjustments, thereby separating the diluent from the solid polymer particles. Additionally, other types of equipment, such as purge columns, separating drums and cyclones, have been used for separation and other downstream treatment.
Efficient slurry polymerization processes typically recycle as much diluent as they reasonably can. That is, as the diluent is vaporized, diluent is processed for recycling back to the loop reactor. As the liquid diluent is vaporized at various stages of the polymerization process after the slurry is withdrawn from the loop reactor, various compounds and contaminants may also be vaporized along with the diluent.
“Heavies” are liquid components heavier than the diluent, and are often vaporized along with the diluent. During the recycling process, a vapor stream containing the diluent and heavies may be condensed and returned to the loop reactor as a recycle stream of the diluent and heavies. Heavies may concentrate or accumulate within the loop reactor and decrease reactor efficiency and/or cause damage within the loop reactor. For example, heavies buildup within the reactor may degrade product quality and/or distort reactor control making reactor problems and blockage more likely.
Another recycling technique separates a slip stream comprising diluent and minor amounts of monomer from the bulk of the flashed fluid components. The slip stream is subjected to olefin removal to give an essentially olefin-free stream for recycle to a catalyst preparation area. The slip stream may be taken off before and/or after a diluent recycle cooler. The slip stream(s) is then passed to a heavies removal column. The heavies removal column removes heavies from the slip stream, which can then be used to prepare a catalyst mud or slurry. The bulk of monomer-containing recycle diluent is passed to a diluent recycle surge (storage) vessel, and a recycle stream is passed back to the reactor. If desired, other purification steps such as water removal can be carried out on this recycle stream.
An efficient system and process of recycling diluent in a slurry polymerization process are desired. Further, an improved heavies removal system and process, which remove heavies from the diluent to be recycled to the reactor, are desired.